Electrical symbols, what does a dot before a triangle mean?

Just another way of drawing a logic inverter ("NOT").

The triangle-with-bubble is an inverter: logic high input yields logic low output, and vice versa. Usually the bubble is shown on the output, but in a mixed-logic system, the bubble can be shown on the input instead.

When the bubble is on the input instead of the output, that indicates that the input is an active-low input.

It indicates an active-low input, active-low means you must apply 0V to make gate working.

Here you can find good explanation

The “bubble” (o) present at the end of the NOT gate symbol above denotes a signal inversion (complementation) of the output signal. But this bubble can also be present at the gates input to indicate an active-LOW input. This inversion of the input signal is not restricted to the NOT gate only but can be used on any digital circuit or gate as shown with the operation of inversion being exactly the same whether on the input or output terminal. The easiest way is to think of the bubble as simply an inverter.